In the manufacture of nuclear fuel pellets, uranium dioxide powder typically is fed into a rotary press. The press consists of a plurality of press units rotatable about a vertical axis. Each press unit includes a die opening and a press table and cooperating upper and lower punches which are slidable into an extended, inward position in the die openings. Each press unit is advanced successively into a punching position where the upper and lower punches are positioned in the extended, inward position in the die opening for pressing material into a compact body. Force generating means, such as upper and lower pneumatic compensators engaging upper and lower rollers between which the punches pass, is positioned at the punching position for exerting a punching force onto upper and lower punches advanced into the punching position. The force generating means acts similar to a large adjustable spring exerting downward force onto the punches advanced into the punching position. When the punching force is exerted on punches advanced into the punching position, the punches are displaced outward. The greater the amount of force exerted against the punches, the less the displacement and the greater the powder compression. This punch displacement is compared with a theoretical "zero" displacement. Based upon this comparison, the amount of powder inserted into the die openings, and the amount of pressure applied at the punching position is varied to obtain a desired pellet length and density. Additionally the punch force is changed to assure an equal displacement of both top and bottom punches. Unequal punch displacement creates a pellet having a diameter varying along the length of the pellet.
It has been determined, however, that the comparing of the punching position displacement with the theoretical "zero" displacement does not always give an actual punch displacement. As a result, after the punching position pressure is adjusted in an attempt to obtain equal punch displacement, the upper and lower punches may appear to displace an equal amount. The inexactitude of punch displacement created by comparing the punching position displacement with the changing datum reference of the theoretical "zero" displacement could result in unequal upper and lower punch displacement even though punch displacement appears to be equal based upon the comparison.
As an example of one type of press machine which suffers the above problem, each press unit includes upper and lower punches having bearings positioned on punch heads which engage a cam surface. As the punch heads ride along the cam surface, the punches are successively advanced into a punching position where the upper and lower punches are positioned in the die opening for pressing material into a more compact body.
Each cam includes upper and lower inclined cam surfaces positioned adjacent the punching position for guiding respective upper and lower punches into the extended, forward position within the die openings A substantially flat cam surface extends from each of the inclined cam surfaces to the punching position for engaging and maintaining the punches in the extended, forward position as the punches approach the punching position. In this extended position, the punches are forced into the die opening and compact the powder to an almost finished pellet size and dimension. The final punching force exerted on the punches presses the powder into the desired dimension and density. The desired pellet length and density are controlled by the amount of powder entering the die and the force applied onto the punches at the punching position.
As the upper and lower punches enter the punching position, the punches pass between two rollers having a pneumatic compensator connected to each roller for exerting a punching force onto each punch. The pneumatic compensator acts similar to an adjustable spring for maintaining pressure on the two rollers. The rollers are positioned so that as the punches pass therebetween, the punches engage the roller periphery forcing the punches downward into a final, compressive position. Punch displacement can be a few millimeters; however, the generated forces resulting from the displacement are tremendous causing compression of the pellet into a final dimension and density.
During powder compression, the powder exerts an equal and opposite effect on the punches and rollers causing an outward displacement of the punches. By adjusting the pressure of the pneumatic compensator, the total force exerted on the rollers and punches is varied and, thus, the punch displacement can be changed. A greater pressure exerted on the rollers and punches creates less rearward displacement of the punches and rollers, and, as a result, the pellets are compressed a greater amount.
A displacement transducer is mounted on each roller and monitors displacement of the punches. The transducers emit a voltage signal corresponding to the displacement of the punches and rollers. At the point of maximum powder compression where the roller contacts the top center of the punch heads, a controller senses and records the transducer voltage output. The voltage output is compared with a reference datum voltage set at electrical "zero" which corresponds theoretically to a point where no punching force has been applied. The difference between the two voltages represents theoretically the punch displacement.
As noted before, this prior art system has drawbacks. The theoretical "zero" position of the transducer shifts because the transducers do not remain stationary. The vibration resulting from high speed rotary press operation, the variations in temperature, unaccurate mechanical machine tolerances and other factors causes the transducers to move slightly. Transducer movement variations in fractions of a millimeter can cause the transducer to shift. Thus, the resulting voltage output from the transducer in a "true" nonpunching position where the punches are positioned in the extended, forward position just prior to application of the punching force will sometimes be above or below the "zero" theoretical voltage. As a result, the transducer voltage output at the point of maximum punch displacement is rarely compared to the "true" zero position and the actual displacement of the punch is not measured accurately.
As a result of inaccurate punch displacement measurement, the finished pellets vary in length from each other because pellet length is a function of punch displacement. Additionally, unequal deflection results in production of irregularly shaped pellets. As noted before, the amount of punch displacement is controlled by adjusting the pressure of the compensator. However, because the actual punch displacement measurement is inaccurate when compared with the electrical "zero", the actual displacement of upper and lower punches sometimes is not equal and irregularities in pellet configuration occur.